scala.collection.compat.BuildFrom.scala Maven / Gradle / Ivy

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/*
 * Scala (https://www.scala-lang.org)
 *
 * Copyright EPFL and Lightbend, Inc.
 *
 * Licensed under Apache License 2.0
 * (http://www.apache.org/licenses/LICENSE-2.0).
 *
 * See the NOTICE file distributed with this work for
 * additional information regarding copyright ownership.
 */

package scala.collection.compat

import scala.collection.generic.CanBuildFrom
import scala.collection.mutable

/** Builds a collection of type `C` from elements of type `A` when a source collection of type `From` is available.
 * Implicit instances of `BuildFrom` are available for all collection types.
 *
 * @tparam From Type of source collection
 * @tparam A Type of elements (e.g. `Int`, `Boolean`, etc.)
 * @tparam C Type of collection (e.g. `List[Int]`, `TreeMap[Int, String]`, etc.)
 */
trait BuildFrom[-From, -A, +C] extends Any {
  def fromSpecific(from: From)(it: IterableOnce[A]): C

  /** Get a Builder for the collection. For non-strict collection types this will use an intermediate buffer.
   * Building collections with `fromSpecific` is preferred because it can be lazy for lazy collections. */
  def newBuilder(from: From): mutable.Builder[A, C]

  @deprecated("Use newBuilder() instead of apply()", "2.13.0")
  @`inline` def apply(from: From): mutable.Builder[A, C] = newBuilder(from)
}

object BuildFrom {

  // Implicit instance derived from an implicit CanBuildFrom instance
  implicit def fromCanBuildFrom[From, A, C](
      implicit cbf: CanBuildFrom[From, A, C]): BuildFrom[From, A, C] =
    new BuildFrom[From, A, C] {
      def fromSpecific(from: From)(it: IterableOnce[A]): C = (cbf(from) ++= it).result()
      def newBuilder(from: From): mutable.Builder[A, C] = cbf(from)
    }

  // Implicit conversion derived from an implicit conversion to CanBuildFrom
  implicit def fromCanBuildFromConversion[X, From, A, C](x: X)(
      implicit toCanBuildFrom: X => CanBuildFrom[From, A, C]): BuildFrom[From, A, C] =
    fromCanBuildFrom(toCanBuildFrom(x))

}